...is that neither side really wants the Bush middle class tax cuts. In 2001 Republicans really wanted to cut taxes for high-income earners, either out of a belief in the enormous sensitivity of high-income earners to marginal tax rates or a deep-seated moral opposition to the principle of progressive taxation. But in order to make those cuts salable, they paired them with tax cuts that also benefited middle-class taxpayers.
In 2004 and 2008, Democrats -- who objected to both the regressive effects of those tax cuts and the revenue-draining effects -- decided to tack to the middle by embracing the middle-class tax cuts and opposing only the cuts that benefit just the top 2%.
But the truth is that Democrats would rather just eliminate the whole thing and have the revenue. And Republicans think the middle-class tax cuts are economically useless. (AEI's Alan Viard, whose resume I accidentally inflated, makes the case against the middle-class tax cuts.) You can also see the thinking in retiring Republican Senator George Voinovich's opposition to extending any of the tax cuts, something you haven't seen from non-retiring GOP Senators.
So basically, neither side wants the middle-class tax cuts. Yet both sides are working to pose as champions of those tax cuts because they're extremely possible. One point of President Obama's post-election deficit commission is to create some way for both parties to agree to to claw back those tax cuts, in part or in whole, in such a way as to insulate both from blame.
In the meantime, the failure of Democrats to put Republicans on the spot by holding a straight vote on the middle-class tax cuts remains baffling. If you're going to compromise your preferences for political expedience, why not at least enjoy the political expedience? If Republicans block the bill, you can blame them both for raising taxes on the middle class out of deference to their rich buddies, which is true, while secretly enjoying the benefits of tax revenues sufficient to fund the federal government and stave off a financial crisis in a few years.